Pixel structure

ABSTRACT

A pixel structure including a control unit, an organic electro-luminescent (OEL) unit, and a filter structure is provided. The control unit is disposed on a substrate and is driven by a scan line and a data line. The OEL unit is disposed on the substrate, and includes a transparent electrode, a light-emitting layer, and a metal electrode. The transparent electrode is electrically connected with the control unit, and the light-emitting layer and the metal electrode are sequentially placed on the transparent electrode. The filter structure is sandwiched between the substrate and the OEL unit, and the filter structure includes a plurality of the first and second dielectric layers. The first and second dielectric layers are alternately stacked, and the refractive index of the first dielectric layers is different from that of the second dielectric layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a pixel structure. More particularly, the present invention relates to the pixel structure of an active matrix organic electro-luminescent display.

2. Description of Related Art

Information technology has become a mainstream industry in our society, thanks to the development of various portable communication and display products. Because the display panel is an indispensable communication interface for man to acquire information, its development is particularly important. Among the display panels, organic electro-luminescent display (OELD) has the greatest potential to become the major display product in the next generation, with the advantages including self-illuminating screen, wide viewing angle, low power consumption, simple manufacturing process, low cost, a wide operating temperature range, a high response speed and full-color display.

The organic electro-luminescent display (OELD) utilizes the self-illuminating property of the organic light-emitting material to display an image. According to the molecular weight of the organic light-emitting material, the OELD panel can be classified into small molecule organic electro-luminescent display (SM-OELD) and polymer electro-luminescent display (PELD). The light-emitting structure of both types of OELD comprises a pair of electrodes and an organic material layer sandwiched between the two electrodes. When a DC voltage is applied to the electrodes, holes are injected from the anode into the organic light-emitting material layer while electrons are injected from the cathode into the organic light-emitting material layer. Due to the electric potential difference produced by an external electric field, hole and electron carriers moving inside the organic light-emitting material layer may collide and trigger radiative recombination. A portion of the energy released by the recombination of the electron and hole may excite the organic light-emitting molecules into an excited state. When the excited molecule releases its energy and returns to a ground state, a definite portion of the energy is released as photons to emit light. Accordingly, the organic electro-luminescent display (OELD) panel emits light following this principle.

FIG. 1A is a cross-sectional view showing a pixel structure of a conventional active matrix-type OLED. FIG. 1B shows a relationship between the wavelength and transmittance of the light-emitting layer shown in FIG. 1A. Please refer to FIG. 1A, the conventional pixel structure 100 is controlled by a scan line and a data line arranged on a substrate 110. The pixel structure 100 comprises a plurality of amorphous silicon thin film transistors (a-Si TFT) 120 a and 120 b, a plurality of dielectric layers 130 and 140, an organic electro-luminescent unit 150 and a pixel define layer 160. The a-Si TFT 120 a and 120 b are arranged on the substrate 110 and are electrically connected with each other, in order to control the organic electro-luminescent unit 150. The dielectric layer 140 made of silicon nitride is arranged over the a-Si TFT 120 a and 120 b. Besides, the organic electro-luminescent unit 150 and the pixel define layer 160 are arranged on the dielectric layer 140 respectively.

More specifically, the a-Si TFT 120 a and 120 b comprise gate electrodes 122 a and 122 b, channel layers 124 a and 124 b, source/drain electrodes 126 a and 126 b, respectively. The gate electrodes 122 a and 122 b are arranged on the substrate 110. The dielectric layer 130 arranged on the substrate 110 covers the gate electrodes 122 a and 122 b. The channel layers 124 a and 124 b are disposed on the dielectric layer 130. The source/drain electrodes 126 a and 126 b are arranged on the channel layers 124 a and 124 b, respectively. In addition, the organic electro-luminescent unit 150 comprises a transparent electrode 152, a light-emitting layer 154 and a metal electrode 156. The transparent electrode 152 is electrically connected to the a-Si TFT 120 b. The light-emitting layer 154 and the metal electrode 156 are sequentially arranged on the transparent electrode 152.

As shown in FIGS. 1A and 1B, a light emitted from the light-emitting layer 154 travels through the transparent electrode 152, the dielectric layer 130 and 140 and the substrate 110 toward the outside of the OLED. As shown in FIG. 1B, the average transmittance of red light (640 nm), green light (515 nm) and blue light (470 nm) in the conventional pixel structure is about 97%. So, the dielectric layers 130 and 140 have little influence on the light emitted from the organic electro-luminescent unit 150. In other words, the display quality of the active matrix-type OLED having the conventional pixel structure 100 is restricted by the characteristics of the organic electro-luminescent unit 150. Compared with the a-Si TFT 120 a and 120 b, a low temperature poly silicon (LTPS) TFT has the advantages of higher carrier mobility and lower operation voltage, thus another pixel structure applied in the active matrix-type OLED has been developed.

FIG. 2A is a cross-sectional view showing another pixel structure applied in a conventional active matrix-type OLED. FIG. 2B shows the relationship between the wavelength and transmittance of the emitting layer shown in FIG. 2A. Please refer to FIG. 2A, the pixel structure 200 is controlled by a scan line and a data line arranged on a substrate 210. The conventional pixel structure 200 comprises a plurality of LTPS thin film transistors 230 a and 230 b, a plurality of dielectric layers 220, 240, 250 and 260, an organic electro-luminescent unit 150 and a pixel define layer 160. The LTPS thin film transistors 230 a and 230 b are electrically connected with each other. The organic electro-luminescent unit 150 is controlled by the LTPS thin film transistors 230 a and 230 b. The dielectric layer 220 is arranged on the substrate 210. Then, the LTPS thin film transistors 230 a and 230 b are arranged on the dielectric layer 220. Besides, the dielectric layer 260 is over the LTPS thin film transistors 230 a and 230 b. The organic electro-luminescent unit 150 and the pixel define layer 160 are arranged on the dielectric layer 260 respectively.

More specifically, the LTPS thin film transistors 230 a and 230 b comprise gate electrodes 232 a and 232 b, channel layers 234 a and 234 b, source/drain electrodes 236 a and 236 b respectively. The channel layers 234 a and 234 b are disposed on the dielectric layer 220. The dielectric layer 220 could prevent the metal ions inside the substrate 210 from diffusing to the channel layers 234 a and 234 b. The dielectric layer 240 is disposed over the channel layers 234 a and 234 b, and the source/drain electrodes 236 a and 236 b are arranged above the channel layers 234 a and 234 b respectively. It should be noted that the gate electrodes 232 a and 232 b are arranged on the dielectric layer 240, and the LTPS thin film transistors 230 a and 230 b are both dual-gate thin film transistor structures. But the LTPS thin film transistors 230 a and 230 b can also be single gate thin film transistor structures. The dielectric layer 250 is arranged over the gate electrodes 232 a and 232 b. The dielectric layer 260 is arranged on the dielectric layer 250.

As shown in FIGS. 2A and 2B, a light emitted from the light-emitting layer 154 travels through the transparent electrode 152, the dielectric layer 260, 250, 240 and 220, and the substrate 210 toward the outside of the OLED. The dielectric layers 260 and 250 are made of silicon nitride, and the dielectric layer 240 is made of silicon dioxide. The material of dielectric layer 240 can be silicon dioxide/silicon nitride. As shown in FIG. 2B, the transmittance of a specific light (500 nm) and a red light (640 nm) with respect to the conventional pixel structure 200 are about 96% and 83% respectively. That is, the luminance characteristics of the organic electro-luminescent unit 150 will be affected by the dielectric layers 240 and 220.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a pixel structure having better color purity.

The present invention is also directed to providing a pixel structure having higher light intensity and better color purity.

According to an embodiment of the present invention, a pixel structure controlled by a scan line and a data line arranged on a substrate is provided. The pixel structure comprises a control unit, an OEL unit and a filter structure. The control unit is arranged on the substrate and is driven by the scan line and the data line. The OEL unit is arranged on the substrate and includes a transparent electrode, a light-emitting layer and a metal electrode. The transparent electrode is electrically connected with the control unit, and the light-emitting layer and the metal electrode are sequentially stacked on the transparent electrode. The filter structure is sandwiched between the substrate and the OEL unit, and it at least comprises a plurality of first and second dielectric layers. The first and second dielectric layers are alternately stacked, and the refractive index of the first dielectric layers is different from that of the second dielectric layers.

According to an embodiment of the present invention, the material of the mentioned first dielectric layers can be silicon nitride or titania, and the material of the second dielectric layers can be silicon dioxide.

According to an embodiment of the present invention, the first dielectric layers and the second dielectric layers are sandwiched between the substrate and the OEL unit, and are extended to the top of the control unit.

According to an embodiment of the present invention, the mentioned filter structure further comprises a third dielectric layer. The third dielectric layer is extended from the bottom of the first and second dielectric layers to the control unit, and it is a single layer or a multi-layer structure. Besides, the filter structure further comprises a fourth dielectric layer over the control unit. The fourth dielectric layer extends below the bottom of the first and second dielectric layers and above the third dielectric layer. The fourth dielectric layer can be a single layer or a multi-layer structure. In addition, the total thickness of the first and second dielectric layers is larger than and equal to that of the third and fourth dielectric layers.

According to an embodiment of the present invention, the mentioned control unit comprises a plurality of active devices. The active devices are electrically connected to each other, and they can be amorphous silicon thin film transistors or low temperature poly-silicon thin film transistors.

According to an embodiment of the present invention, the pixel structure further comprises a pixel define layer arranged upon the control unit and a portion of the transparent electrode.

According to another embodiment of the present invention, a pixel structure driven by a scan line and a data line arranged on a substrate is provided. The pixel structure comprises a control unit, an OEL unit and a semi-transparent reflector structure. The control unit driven by the scan line and the data line is arranged on the substrate. The OEL unit is also arranged on the substrate and comprises a transparent electrode, a light-emitting layer and a metal electrode. The transparent electrode is electrically connected with the control unit. The light-emitting layer is disposed on the transparent electrode, and the metal electrode is disposed on the light-emitting layer. The semi-transparent reflector structure is sandwiched between the substrate and the OEL unit, and it comprises at least a plurality of first and second dielectric layers. The first and second dielectric layers are alternately stacked, and the refractive index of the first dielectric layers is different from that of the second dielectric layers.

According to an embodiment of the present invention, the optical thickness of each mentioned first and second dielectric layers is N times of a quarter of a wavelength, wherein N is an integer.

According to an embodiment of the present invention, the first and second dielectric layers comprise silicon nitride and silicon dioxide respectively.

According to an embodiment of the present invention, the mentioned semi-transparent reflector structure further comprises a middle layer arranged in the middle of the first and second dielectric layers, and the refractive index of the middle layer is larger than that of the second dielectric layers. Besides, the middle layer comprises silicon nitride or titanian.

According to an embodiment of the present invention, the first dielectric layers and the second dielectric layers are sandwiched between the substrate and the OEL unit, and are extended to the top of the control unit.

According to an embodiment of the present invention, the semi-transparent reflector structure further comprises a third dielectric layer. The third dielectric layer extends from the bottom of the first and second dielectric layers to the control unit, and it can be a single layer or multi-layer structure. In addition, the semi-transparent reflector structure further comprises a fourth dielectric layer over the control unit. The fourth dielectric layer extends below the bottom of the first and second dielectric layers and above the third dielectric layer, and it can be a single layer or multi-layer structure. Further, the total thickness of the first and second dielectric layers is larger than and equal to that of the third and fourth dielectric layers.

According to an embodiment of the present invention, the control unit comprises a plurality of active devices, and the active devices are electrically connected to each other. Besides, the active devices can be an amorphous silicon thin film transistor or a low temperature poly-silicon thin film transistor.

According to an embodiment of the present invention, the pixel structure further comprises a pixel define layer arranged above the control unit and a portion of the transparent electrode.

In the present invention, a filter structure or a semi-transparent reflector structure having a plurality of dielectric layers with different refractive indices is sandwiched between the OEL unit and the substrate, to improve the color purity and the light intensity. Because the filter structure and the semi-transparent reflector structure are both multi-dielectric-layer structure, it's easier to planarize the filter structure and the semi-transparent reflector structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1A is a schematic cross-sectional view showing a pixel structure of a conventional active matrix-type OLED.

FIG. 1B shows a relationship of the wavelength and transmittance of the active matrix-type OLED shown in FIG. 1A.

FIG. 2A is a schematic cross-sectional view showing another pixel structure applied to a conventional active matrix-type OLED.

FIG. 2B shows a relationship of the wavelength and transmittance of the emitting layer shown in FIG. 2A.

FIG. 3A is a schematic cross-sectional view showing a pixel structure according to the first embodiment of the present invention.

FIG. 3B shows a relationship of the wavelength and transmittance of the pixel structure shown in FIG. 3A.

FIG. 4A is a schematic cross-sectional view showing a pixel structure according to the second embodiment of the present invention.

FIG. 4B shows a relationship of the wavelength and transmittance of the pixel structure shown in FIG. 4A.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

First Embodiment

FIG. 3A is a schematic cross-sectional view showing a pixel structure according to the first embodiment of the present invention. FIG. 3B shows a relationship of the wavelength and transmittance of the pixel structure shown in FIG. 3A. Please refer to FIG. 3A, the pixel structure 300 of the invention may apply to an active matrix OLED. The pixel structure 300 is controlled by a scan line and a data line arranged on a substrate 310. The pixel structure 300 comprises a control unit 320, an OEL unit 340 and a filter structure 330. The control unit 320 is arranged on the substrate 310 and is driven by the scan line and the data line.

The control unit 320 may comprise a plurality of a-Si TFT 320 a and 320 b, and the a-Si TFT 320 a and 320 b are electrically connected with each other. The a-Si TFT 320 a and 320 b have gate electrodes 322 a and 322 b, channel layers 324 a and 324 b, and source electrode/drain electrode 326 a and 326 b respectively. The gate electrodes 322 a and 322 b are disposed over the substrate 310. The dielectric layer 332 is arranged on the substrate 310 and is over the gate electrodes 322 a and 322 b. The dielectric layer 332 is disposed inside the a-Si TFT 320 a and 320 b and is extended between the OEL unit 340 and the substrate 310. Besides, the dielectric layer 332 can be a single layer or multi-layer structure. The dielectric layer 332 can be a single layer structure made of silicon nitride or a multi-layer structure made of silicon nitride/silicon dioxide. However, the material of the dielectric layer 332 is not limited to silicon nitride or silicon dioxide.

The channel layer 324 a and 324 b are arranged on the dielectric layer 332, and the source electrode/drain electrode 326 a and 326 b are arranged on the channel layer 324 a and 324 b respectively. Specially, the control unit 320 can be composed of a plurality of LTPS thin film transistors or other types of active device. The control unit 320 may have a top gate or a bottom gate. Otherwise, a part of the control unit 320 may have a top gate and others may have a bottom gate. Besides, the structure of the a-Si TFT 320 a and 320 b and electrical connection method between the a-Si TFT 320 a and 320 b are not restricted to what is shown in FIG. 3A. The dielectric layer 334 is disposed over the a-Si TFT 320 a and 320 b and is extended between the OEL unit 340 and the dielectric layer 332. Further, the dielectric layer 334 can be a single layer structure made of silicon nitride or a multi-layer structure made of silicon nitride/silicon dioxide.

The dielectric layer 336 covering the control unit 320 is arranged between the OEL unit 340 and the dielectric layer 334. More specifically, the dielectric layers 336 a and 336 b with different refractive indices are alternately stacked to form the dielectric layer 336. The refractive index of the dielectric layers 336 a is different from that of the dielectric layers 336 b. In addition, the dielectric layers 336 a comprise silicon nitride or titania, and the dielectric layers 336 b comprise silicon dioxide.

It should be noted that the dielectric layers 332, 334 and 336 sandwiched between the OEL unit 340 and the substrate 310 form the filter structure 330. The characteristics of the control unit 320 are controlled by the thickness of the dielectric layer 332 or 334. Therefore, the total thickness of the dielectric layer 336 is larger than or equal to that of the dielectric layers 332 and 334, to achieve a specific filtering effect. Besides, the dielectric layer 336 is composed of six dielectric layers 336 a and six dielectric layers 336 b. The filter effect is better with more dielectric layers of the filter structure 330. In addition, the dielectric layer 336 can comprise multiple dielectric layers with different refractive indices and is not limited to only the dielectric layers 336 a and 336 b with two refractive indices. Further, the stacking sequence of the dielectric layers 336 a and 336 b can be reversed.

The OEL unit 340 comprises a transparent electrode 342, a light-emitting layer 344 and a metal electrode 346. The transparent electrode 342 is electrically connected to the a-Si thin film transistor 320 b. The material of the transparent electrode 342 can be indium tin oxide (ITO), aluminum zinc oxide (AZO), indium zinc oxide (IZO) or other transparent metal oxide. The light-emitting layer 344 and metal electrode 346 are sequentially arranged on the transparent electrode 342. The material of the metal electrode 346 can be Ag, Mg/Ag or other metal with high reflectivity. Besides, the light-emitting layer 344 can be a stacked multi-layer structure. The pixel structure 300 of the invention may further comprise a pixel define layer 350 for separating the OEL units 340. The pixel define layer 350 is arranged on the control unit 320 and a portion of the transparent electrode 342.

Please refer to FIGS. 3A and 3B. Among all kinds of light with different wavelength emitted from the OEL unit 340, the red light (640 nm), the green light (515 nm) and the blue light (470 nm) have better transmittance. In other words, the filter structure 330 having many dielectric layers with different refractive indices can purify the light emitted from the OEL unit 340. Because the dielectric layers 332 and 334 of the filter structure 330 is a part of the a-Si thin film transistor 320 a and 320 b, the characteristics of the a-Si thin film transistor 320 a and 320 b would not change. The dielectric layer 336 of the filter structure 330 is a multi-layer structure, so it is easier to planarize the dielectric layer 336. Then, the subsequently formed transparent electrode 342, light-emitting layer 344 and metal electrode 346 of the OEL unit 340 would have higher planarity.

According to an embodiment of the present invention, after the a-Si thin film transistors 320 a and 320 b are formed, the dielectric layers 332 and 334 above the OEL unit 340 are removed first, and then the dielectric layer 336 composed of a plurality of dielectric layers is formed. In another embodiment of the present invention, only the dielectric layer 332 above the OEL unit 340 is removed, and then the dielectric layer 336 having a plurality of dielectric layers is formed. In another embodiment of the present invention, the dielectric layer 336 composed of a plurality of dielectric layers is formed on the OEL unit 340 after the a-Si thin film transistors 320 a and 320 b are formed.

Second Embodiment

FIG. 4A is a schematic cross-sectional view showing a pixel structure according to the second embodiment of the present invention. FIG. 4B shows a relationship between the wavelength and transmittance of the pixel structure shown in FIG. 4A. Please refer to FIG. 4A. The second embodiment is similar to the first embodiment, but the difference is that the dielectric layers 332, 334 and 412 of the pixel structure 400 are below the OEL unit 340 and form a semi-transparent reflector structure 410. The semi-transparent reflector structure 410 is a distributed bragg reflector (DBR). The total thickness of the dielectric layer 412 can be larger than that of the dielectric layers 332 and 334. Besides, the transparent electrode 342 and the light-emitting layer 344 form a cavity, and the metal electrode 346 becomes a metal mirror while a resonant cavity is formed between the semi-transparent reflector structure 410 and the metal electrode 346. It is noted that the thickness of the transparent electrode 342 varies with the light emitted from the OEL unit 340. For red light, green light and blue light, the thickness of the transparent electrode 342 can be 245 nm, 150 nm and 184 nm respectively.

More specifically, the dielectric layer 412 comprises a plurality of dielectric layers 412 a and 412 b and a middle layer 412 c. The dielectric layers 412 a and 412 b are alternately stacked, and the refractive index of the dielectric layers 412 a is different from that of the dielectric layers 412 b. The dielectric layers 412 a can be made of silicon nitride, and the dielectric layers 412 b can be made of silicon dioxide. The optical thickness of each dielectric layer 412 a and 412 b is a quarter of a wavelength or N times a quarter of a wavelength, wherein N in an integer. In addition, the middle layer 412 c is arranged in the middle of the dielectric layers 412 a and 412 b, to improve the optical characteristics. The refractive index of the middle layer 412 c is larger than that of the dielectric layer 412 b. The middle layer 412 c can be made of silicon nitride or titania. If the middle layer 412 c comprises silicon nitride, the thickness of the middle layer 412 c is about 396 nm. And if the middle layer 412 c is composed of titania, the thickness of the middle layer 412 c is about 350 nm.

As shown in FIGS. 4A and 4B, compared with the light intensity of original red light (R), green light (G) and blue light (B), the red light (R′), green light (G′) and blue light (B′) traveling through the semi-transparent reflector structure 410 have higher light intensity. In other words, the resonant cavity formed between the semi-transparent reflector structure 410 composed of a plurality of dielectric layers with different refractive indices and the metal electrode 346 can not only purify the light emitted from the OEL unit 340 but also increase the light intensity. Therefore, the semi-transparent reflector structure 410 is in accordance with the filter structure of a specific range of the wavelength. Because the dielectric layer 332 of the semi-transparent reflector structure 410 is also a part of the a-Si thin film transistors 320 a and 320 b, the characteristics of the a-Si thin film transistors 320 a and 320 b would not change. Besides, the dielectric layer 412 of the semi-transparent reflector structure 410 is a multi-layer structure, so it is easier to planarize the dielectric layer 412. Then, the subsequently formed transparent electrode 342, light-emitting layer 344 and metal electrode 346 of the OEL unit 340 will have higher surface planarity.

The second embodiment can have various types as the first embodiment. For example, after forming the a-Si thin film transistors 320 a and 320 b, the dielectric layers 332 and 334 above the OEL unit 340 are removed first, and then the dielectric layer 412 composed of a plurality of dielectric layers is formed. Alternatively, only the dielectric layer 332 above the OEL unit 340 is removed, and then the dielectric layer 412 having a plurality of dielectric layers is formed. Otherwise, the dielectric layer 412 composed of a plurality of dielectric layers is formed on the OEL unit 340 after the a-Si thin film transistors 320 a and 320 b are formed.

The dielectric layer 412 can only include the dielectric layers 412 a and 412 b. In the same way, the dielectric layer 412 of the embodiment is composed of four dielectric layers 412 a and four dielectric layers 412 b. The reflection effect is improved with increased number of the dielectric layers of the semi-transparent reflector structure 410. In addition, the stacking sequence of the dielectric layer 412 a and 412 b can be appropriately changed.

In summary, the pixel structure of the invention has at least the following advantages:

1. Compared with the prior art, the filter structure or the semi-transparent reflector structure composed of a plurality of dielectric layers with different refractive indices sandwiched between the substrate and the OEL unit can improve the color purity and light intensity of the light emitted from the OEL unit.

2. The a-Si thin film transistor or the LTPS thin film transistor can be part of the filter structure or the semi-transparent reflector structure without affecting the characteristics of the a-Si thin film transistor or the LTPS thin film transistor.

3. Because the filter structure and the semi-transparent reflector structure are both multi-dielectric-layer structure, so they can be easily planarized. And, the subsequently formed OEL units on the filter structure or the semi-transparent reflector structure will have higher surface planarity.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

1. A pixel structure, arranged on a substrate, comprising: a control unit arranged on the substrate; an OEL unit arranged on the substrate, the OEL unit comprising a transparent electrode, a light-emitting layer, and a metal electrode, wherein the transparent electrode is electrically connected with the control unit, and the light-emitting layer and the metal electrode are sequentially stacked on the transparent electrode; and a filter structure sandwiched between the substrate and the OEL unit, the filter structure at least comprising a plurality of first and second dielectric layers, the first and second dielectric layers being alternately stacked, and the refractive index of the first dielectric layers being different from that of the second dielectric layers, wherein the filter structure further comprises a third dielectric layer, the third dielectric layer extends below the bottom of the first and second dielectric layers to the control unit, and the third dielectric layer is a part of the control unit, the filter structure further comprising a fourth dielectric layer over the control unit, wherein the fourth dielectric layer extends below the bottom of the first and second dielectric layers and above the third dielectric layer, and the first, second, third, and fourth dielectric layers sandwiched between the OEL unit and the substrate form the filter structure, wherein the total thickness of the first dielectric layer and the second dielectric layer is larger than or equal to the total thickness of the third dielectric layer and the fourth dielectric layer.
 2. The pixel structure according to claim 1, wherein a material of the first dielectric layers comprises silicon nitride or titania, and a material of the second dielectric layers comprises silicon dioxide.
 3. The pixel structure according to claim 1, wherein the first dielectric layers and the second dielectric layers are sandwiched between the substrate and the OEL unit, and are extended in a direction toward the control unit and are over the control unit.
 4. The pixel structure according to claim 1, wherein the third dielectric layer is a single layer or a multi-layer structure.
 5. The pixel structure according to claim 1, wherein the fourth dielectric layer is a single layer or a multi-layer structure.
 6. The pixel structure according to claim 1, wherein the control unit comprises a plurality of active devices, and the active devices are electrically connected to each other.
 7. The pixel structure according to claim 6, wherein the active devices comprise amorphous silicon thin film transistors or low temperature poly-silicon thin film transistors.
 8. The pixel structure according to claim 1, further comprising a pixel define layer arranged over the control unit and a portion of the transparent electrode.
 9. A pixel structure, arranged on a substrate, comprising: a control unit arranged on the substrate; an OEL unit arranged on the substrate, the OEL unit comprising a transparent electrode, a light-emitting layer, and a metal electrode, wherein the transparent electrode is electrically connected with the control unit, and the light-emitting layer and the metal electrode are sequentially stacked on the transparent electrode; and a filter structure sandwiched between the substrate and the OEL unit, the filter structure at least comprising a plurality of first and second inorganic dielectric layers, the first and second inorganic dielectric layers being alternately stacked, and the refractive index of the first inorganic dielectric layers being different from that of the second inorganic dielectric layers, wherein the filter structure further comprises a third inorganic dielectric layer, the third inorganic dielectric layer extends below the bottom of the first and second inorganic dielectric layers to the control unit, and the third inorganic dielectric layer is a part of the control unit, the filter structure further comprising a fourth inorganic dielectric layer over the control unit, wherein the fourth inorganic dielectric layer extends below the bottom of the first and second inorganic dielectric layers and above the third inorganic dielectric layer, and the first, second, third, and fourth inorganic dielectric layers sandwiched between the OEL unit and the substrate form the filter structure, wherein the total thickness of the first dielectric layer and the second dielectric layer is larger than or equal to the total thickness of the third dielectric layer and the fourth dielectric layer. 